Exhaust Gas Purification System for Working Machine

ABSTRACT

Disclosed is an exhaust gas purification system for a working machine. The system is provided with a filter for capturing particulate matter contained in exhaust gas from an engine, a differential pressure sensor for detecting a differential pressure between an exhaust upstream side and an exhaust downstream side of the filter, and a controller having a regeneration determination unit for determining whether or not a time, at which forced regeneration is needed, has been reached. The controller includes one that has a variation determination unit for determining whether or not a state quantity relevant to an operation of the engine, for example, an engine speed has varied abruptly and that, when the state quantity is determined to have abruptly varied, performs processing to invalidate the determination by the regeneration determination unit during a predetermined time in which an effect of the state quantity is considered to diminish.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority of Japanese Patent Application2011-004960 filed Jan. 13, 2011, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust gas purification system fora working machine such as a hydraulic excavator, which is provided witha filter for removing particulate matter (hereinafter abbreviated as“PM”) contained in exhaust gas from an engine.

2. Description of the Related Art

As a conventional technology of this type, there is one disclosed inJP-A-2005-307878. This conventional technology is provided with a filterfor capturing PM, which is contained in exhaust gas from an engine, onan exhaust downstream side, an exhaust gas temperature sensor, and adifferential pressure sensor for detecting a differential pressurebetween an exhaust upstream side and the exhaust downstream side of thefilter. The conventional technology is also provided with a computingunit for computing a flow rate of exhaust gas and a regenerationdetermination unit for determining, by a comparison between thedifferential pressure detected at the differential pressure sensor and adeterminative differential pressure as a threshold level fordetermination, whether or not a time, at which forced regeneration isneeded to burn PM captured on the filter, has been reached.

According to the conventional technology having such a constitution asdescribed above, a differential pressure ΔP1 and a determinativedifferential pressure ΔP2 are compared with each other at theregeneration determination unit of the controller and, when ΔP1>ΔP2,forced regeneration is determined to be needed. The differentialpressure ΔP1 is determined by converting a detected differentialpressure ΔP, which is detected at the differential pressure sensor, to acorresponding value at a standard temperature of exhaust gas from acorrelation between a temperature of exhaust gas as detected at theexhaust gas temperature sensor and the standard temperature. Thedeterminative differential pressure ΔP2, on the other hand, is athreshold level at the standard temperature, which corresponds to a flowrate of exhaust gas as computed at the computing unit of the controller.

Forced regeneration is to inject fuel into exhaust gas from an enginesuch that using an oxidation reaction by an oxidation catalyst, thetemperature of exhaust gas is raised to burn off PM deposited on afilter. Clogging of the filter can, therefore, be solved by forcedregeneration.

With the above-described conventional exhaust gas purification system,an appropriate value can be calculated as the above-mentioneddeterminative differential pressure ΔP insofar as it is applied to avehicle, such as a truck, that does not undergo much abrupt variationsin the injection quantity of fuel or abrupt variations in the volume ofintake air, because the flow rate of exhaust gas remains stable duringan operation. In a working machine, such as a hydraulic excavator, forwhich the present invention is useful, however, its body undergoesfrequent abrupt variations such as variations in load and variations inswing torque so that the flow rates of exhaust gas as calculated at thetime of the respective variations also vary significantly. As aconsequence, no appropriate determinative differential pressure ΔP2 maybe calculated in some instances. When the conventional technology isapplied to a working machine and a determination is made at theregeneration determination unit of the controller by using such adeterminative differential pressure ΔP2, a problem may hence arise that,even if PM has not deposited much on the filter actually, ΔP1>ΔP2 isdetermined and forced regeneration is performed although it is notneeded. Such unnecessary forced regeneration results in a wastefulinjection of fuel, and leads to a deterioration in fuel economy.

SUMMARY OF THE INVENTION

With the foregoing circumstances of the above-described conventionaltechnology in view, the present invention has as an object thereof theprovision of an exhaust gas purification system for a working machine,which can realize forced regeneration without being affected by abruptvariations of a body.

To achieve the above-described object, the present invention provides,in one aspect thereof, an exhaust gas purification system for a workingmachine provided with working equipment, a main body with the workingequipment attached thereto, and an engine arranged on the main body todrive the working equipment, said exhaust gas purification system beingprovided with a filter for capturing particulate matter, which iscontained in exhaust gas from the engine, on an exhaust downstream side,a differential pressure sensor for detecting a differential pressurebetween an exhaust upstream side and the exhaust downstream side of thefilter, and a controller having a regeneration determination unit fordetermining, by a comparison between the differential pressure detectedat the differential pressure sensor and a determinative differentialpressure as a threshold level for determination, whether or not a time,at which forced regeneration is needed to burn the particulate mattercaptured on the filter, has been reached, wherein the controllercomprises one that has a variation determination unit for determiningwhether or not a state quantity relevant to an operation of the enginehas varied abruptly and that, when the state quantity is determined tohave abruptly varied by the variation determination unit, performsprocessing to invalidate the determination by the regenerationdetermination unit during a predetermined time in which an effect of thestate quantity is considered to diminish.

The present invention has been made with an attention focused on thefact that upon occurrence of an abrupt variation on a body, a statequantity relevant to an operation of an engine, such as the engine speedor the injection quantity of fuel, varies abruptly. According to thepresent invention, when the state quantity relevant to the operation ofthe engine is determined by the variation determination unit of thecontroller to have abruptly varied in response to an abrupt variation ofthe body, processing is performed by the controller to invalidate thedetermination by the regeneration determination unit that determineswhether or not forced regeneration is to be performed, in other words,to terminate a determination function of the regeneration determinationunit during a predetermined time in which an effect of the abruptvariation in the state quantity is considered to diminish. Theabove-described predetermined time can be set experimentally orempirically in view of load variations which may occur on the associatedworking machine. As a consequence, the present invention can realizeforced regeneration without being affected by abrupt variations of thebody.

The controller may preferably have a first computing unit for computinga flow rate of exhaust gas, and a second computing unit for computingthe determinative differential pressure based on the flow rate ofexhaust gas as computed at the first computing unit and a map preset inthe controller and indicating correlations between flow rates of exhaustgas and determinative differential pressures.

Preferably, the exhaust gas purification system may be further providedwith a fuel control unit for controlling an injection quantity of fuelto be fed to the engine, an intake air volume sensor for detecting avolume of intake air to be fed to the engine and outputting a detectionsignal to the controller, an intake air temperature sensor for detectinga temperature of intake air and outputting a detection signal to thecontroller, and an exhaust gas temperature sensor for detecting atemperature of exhaust gas from the engine and outputting a detectionsignal to the controller; the controller may further comprise a fuelinjection quantity instruction unit for outputting an instruction signalto instruct the injection quantity of fuel to the fuel control unit, andan intake air weight computing unit for computing a weight of intake airbased on a density of intake air, which is determined according to thetemperature of intake air as detected at the intake air temperaturesensor and a map preset in the controller and indicating correlationsbetween intake air temperatures and intake air densities, and the volumeof intake air as detected at the intake air volume sensor; an exhaustgas weight computing unit for computing a weight of exhaust gas based onthe weight of intake air as computed at the intake air weight computingunit and the injection quantity of fuel as instructed by the fuelinjection quantity instruction unit; and the first computing unit of thecontroller may perform processing to compute a flow rate of exhaust gasbased on a density of exhaust gas, which is determined according to thetemperature of exhaust gas as detected at the exhaust gas temperaturesensor and a map preset in the controller and indicating correlationsbetween exhaust gas temperatures and exhaust gas densities, and theweight of exhaust gas as computed at the exhaust gas weight computingunit.

The exhaust gas purification system may preferably be further providedwith an engine speed sensor for detecting a revolution speed of theengine and outputting a detection signal to the controller; and thestate quantity relevant to the operation of the engine may preferably beat least one of the revolution speed of the engine as detected at theengine speed sensor, the injection quantity of fuel as instructed by thefuel injection quantity instruction unit, the volume of intake air asdetected at the intake air volume sensor, and the flow rate of exhaustgas as computed at the first computing unit of the controller.

In the exhaust gas purification system of the present invention for theworking machine equipped with the working equipment, the controller isconstituted to comprise one that has the variation determination unitfor determining whether or not the state quantity relevant to anoperation of the engine has varied abruptly and that, when the statequantity is determined to have abruptly varied by the variationdetermination unit, performs processing to invalidate the determinationby the regeneration determination unit during the predetermined time inwhich the effect of the state quantity is considered to diminish. Owingto this constitution, it is possible to realize forced regenerationwithout being affected by abrupt variations of the body. Describedspecifically, it is possible to minimize the performance of unnecessaryforced regeneration that would otherwise tend to be performed inresponse to abrupt variations of the body, thereby making it possible toprevent wasteful injections of fuel and hence to improve the fueleconomy of the working machine equipped with the exhaust gaspurification system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing a hydraulic excavator described as anexample of a working machine in which an exhaust gas purification systemaccording to one embodiment of the present invention can be arranged.

FIG. 2 is a diagram illustrating the constitution of the exhaust gaspurification system according to the embodiment as arranged in thehydraulic excavator shown in FIG. 1.

FIG. 3 is a block diagram depicting an essential constitution of acontroller included in the embodiment.

FIG. 4 is a diagram illustrating characteristics available from theembodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The exhaust gas purification system according to the one embodiment ofthe present invention for the working machine will hereinafter bedescribed based on the drawings.

As shown in FIG. 1, the hydraulic excavator which makes up the workingmachine is provided with a travel base 1 and an upperstructure 2 mountedon the travel base 1. These travel base 1 and upperstructure 2 make up amain body. This hydraulic excavator is also provided with workingequipment 3 attached tiltably in up-and-down directions to theupperstructure 2 and including a boom, an arm and so on, an operator'scab 4 arranged on the upperstructure 2, a counterweight 5 for assuring aweight balance, and an engine compartment 6 arranged between theoperator's cab 4 and the counterweight 5.

This hydraulic excavator is also provided, as illustrated in FIG. 2,with an engine 10 accommodated in the engine compartment 6, an aircleaner 11 for removing dust from air to be inducted into the engine 10,that is, from intake air, an air compressor 12 of a turbocharger forcompressing the intake air cleaned by the air cleaner 11 and guidedthrough an intake air passage 30, intake air passages 31,32 for guidinginto the engine 10 the intake air compressed by the compressor 12, andan air cooler 13 arranged between the intake air passage 31 and theintake air passage 32 for cooling the intake air to be fed to the engine10. The hydraulic excavator is further provided with a turbine 14 of theturbocharger and an exhaust gas passage 33, both of which guide exhaustgas from the engine 10, recirculation passages 34,35 for recirculating aportion of exhaust gas from the engine 10 and feeding it again into theengine 10, and an EGR (Exhaust Gas Recirculation) cooler 15 arrangedbetween these recirculation passages 34 and 35 for cooling the exhaustgas to be fed to the engine 10.

The exhaust gas purification system of this embodiment for the hydraulicexcavator of such a constitution as described above is provided, as alsoillustrated in FIG. 2, provided with a filter 20 for capturing PM, whichis contained in the exhaust gas from the engine 10, at an exhaustdownstream side and a differential pressure sensor 21 for detecting adifferential pressure between an exhaust upstream side and the exhaustdownstream side of the filter 20. This embodiment is also provided witha controller 22 having a regeneration determination unit 22 a depictedin FIG. 3. By a comparison between the differential pressure detected atthe differential pressure sensor 21, namely the detected differentialpressure ΔP and a determinative differential ΔPo as a threshold levelfor determination, the regeneration determination unit 22 a determineswhether or not a time, at which forced regeneration is needed to burn PMcaptured on the filter 20, has been reached. When it is determined atthe regeneration determination unit 22 a that the time, at which forcedregeneration is needed, has been reached, a control signal is outputtedfrom the controller 22 to a fuel injector 28, and by this fuel injector28, a predetermined quantity of fuel is injected to mix it into exhaustgas from the engine 10.

Referring back to FIG. 2, this embodiment is also provided with anintake air volume sensor 23, an intake air temperature sensor 24 and anexhaust gas temperature sensor 27. The intake air volume sensor 23detects a quantity of air guided into the intake air passage 30, thatis, a quantity of intake air, and outputs a detection signal to thecontroller 22. The intake air temperature sensor 24 detects atemperature of the intake air, and outputs a detection signal to thecontroller 22. The exhaust gas temperature sensor 27 detects atemperature of exhaust gas guided into the exhaust gas passage 33, andoutputs a detection signal to the controller 22.

This embodiment is further provided, as depicted in FIG. 3, with a fuelcontrol unit 25 and an engine speed sensor 26. The fuel control unit 25controls a quantity of fuel, which is to be fed to the engine 10,response to a control signal outputted from a fuel injection quantityinstruction unit 22 e included in the controller 22. The engine speedsensor 26 detects a revolution speed of the engine 10, and outputs adetection signal to the controller 22.

In this embodiment, the controller 22 includes, as depicted in FIG. 3,one that has a variation determination unit 22 b for determining whetheror not a state quantity relevant to an operation of the engine 10, forexample, an engine speed detected at the engine speed sensor 26 hasvaried abruptly, and that, when the engine speed is determined to haveabruptly varied by the variation determination unit 22 b, performsprocessing to invalidate the above-mentioned determination by theregeneration determination unit 22 a during a predetermined time inwhich an effect of the abrupt variation in engine speed is considered todiminish.

A determinative revolution speed variation ΔN1 as a threshold level forthe determination of an abrupt variation in engine speed is stored inthe controller 22. The variation determination unit 22 b compares thedeterminative revolution speed variation ΔN1 with an actual revolutionspeed variation ΔN computed based on the detection value by the enginespeed sensor 26 and, when ΔN>ΔN1, determines that the engine speed hasvaried abruptly.

The above-mentioned predetermined time can be set experimentally orempirically in view of variations of the body, such as variations inload and variations in swing torque, which can occur on the hydraulicexcavator shown in FIG. 1.

The controller is also provided, as depicted in FIG. 3, with a firstcomputing unit 22 c and a second computing unit 22 d. The firstcomputing unit 22 c computes an exhaust gas flow rate Vex. The secondcomputing unit 22 d computes a determinative differential pressure ΔPobased on the exhaust gas flow rate Vex computed at the first computingunit 22 c and a map preset in the controller 22 and indicatingcorrelations between exhaust gas flow rates and determinativedifferential pressures.

The controller is further provided with an intake air weight computingunit 22 f, which computes an intake air weight Gin(=f2×Vin) based on aintake air density f2, which is determined according to an intake airtemperature Tin detected at the intake air temperature sensor 24 and amap preset in the controller 22 and indicating correlations betweenintake air temperatures and intake air densities, and an intake airvolume Vin detected at the intake air volume sensor 23.

The controller 22 is still further provided with an exhaust gas weightcomputing unit 22 g for computing an exhaust gas weight Gex(=Gin+q)based on the intake air weight Gin computed at the intake air weightcomputing unit 22 f and a fuel injection quantity q instructed by thefuel injection quantity instruction unit 22 e.

The above-mentioned first computing unit 22 c of the controller 22performs processing to compute an exhaust gas flow rate Vex(=Gex/f3)based on an exhaust gas density f3 and the exhaust gas weight Gexcomputed at the exhaust gas weight computing unit 22 g. The exhaust gasdensity f3 is determined according to an exhaust gas temperature Texdetected at the exhaust gas temperature sensor 27 and a map preset inthe controller 22 and indicating correlations between exhaust gastemperatures and exhaust gas densities. Based on the determinativedifferential pressure ΔPo computed at the second computing unit 22 d inaccordance with the exhaust gas flow rate Vex computed at the firstcomputing unit 22 c and the detected differential pressure ΔP detectedat the differential pressure sensor 21, the above-mentioned regenerationdetermination unit 22 a determines, as mentioned above, whether or notthe time, at which forced regeneration is needed, has been reached.

According to this embodiment constituted as described above, when asindicated by a sensing range S1 in FIG. 4, the load A is relativelystable and the engine speed N is maintained at a constant highrevolution speed, the actual revolution speed variation ΔN calculatedbased on the detection value detected at the engine speed sensor 26 isnot higher than the determinative revolution speed variation ΔN1, thatis, ΔN≦ΔN1, so that at the variation determination unit 22 b of thecontroller 22, the engine speed is determined to have undergone noabrupt variation. Therefore, the regeneration determination unit 22 afunctions normally, and performs a determination as to whether or notthe time, at which forced regeneration is needed to burn PM captured onthe filter 20, has been reached, specifically a determination thatcompares the detected differential pressure ΔP and the determinativedifferential pressure ΔPo with each other.

When ΔP≦ΔPo is determined at the regeneration determination unit 22 a,it is determined that the time, at which forced regeneration is needed,has not been reached. As a consequence, no control signal is outputtedto activate the fuel injector 28. When ΔP>ΔPo is determined at theregeneration determination unit 22 a, on the other hand, it isdetermined that the time, at which forced regeneration is needed, hasbeen reached, and the fuel injector 28 performs an injection to mix fuelin the exhaust gas from the engine 10 as mentioned above. As a result,the temperature of the exhaust gas rises under the action of theoxidation catalyst, the PM captured on the filter 20 burns, and theclogging of the filter 20 is solved.

When the load A has undergone an abrupt variation and the engine speedhas undergone, for example, an abrupt drop, both at the variationdetermination unit 22 b of the controller 22, as indicated by a sensingrange S2 in FIG. 4, an actual revolution speed variation ΔN calculatedbased on a detection value of the engine speed sensor 26 becomes greaterthan the determinative revolution speed variation ΔN1, that is, ΔN>ΔN1is obtained, processing is performed at the controller 22 to invalidatethe determination processing by the regeneration determination unit 22 aduring a predetermined time T in which the effect of the rapid variationin engine speed is considered to diminish.

According to this embodiment constituted as described above, it ispossible to perform forced regeneration without being affected by abruptvariations in the engine speed N, in other words, without being affectedby abrupt variations of the upperstructure 2 or travel base 1. As aconsequence, it is possible to minimize the practice of unnecessaryforced regeneration, which would otherwise tend to be performed inresponse to abrupt variations of the upperstructure 2 or travel base 1and to prevent wasteful injections of fuel. Owing to this feature, it ispossible to improve the fuel economy of a hydraulic excavator equippedwith an exhaust gas purification system.

In the above-described embodiment, the engine speed N is described asthe state quantity relevant to the operation of the engine 10 to bedetermined at the variation determination unit 22 b of the controller22. It is, however, to be noted that this state quantity can be the fuelinjection quantity q instructed by the fuel injection quantityinstruction unit 22 e of the controller 22, the intake air volume Vindetected at the intake air volume sensor 23, the exhaust air flow rateVex computed at the first computing unit 22 c, or the like. It is alsoto be noted that the exhaust gas purification system may be designed todetermine plural ones of these state quantities at the variationdetermination unit 22 b.

1. An exhaust gas purification system for a working machine providedwith working equipment, a main body with the working equipment attachedthereto, and an engine arranged on the main body to drive the workingequipment, said exhaust gas purification system being provided with afilter for capturing particulate matter, which is contained in exhaustgas from the engine, on an exhaust downstream side, a differentialpressure sensor for detecting a differential pressure between an exhaustupstream side and the exhaust downstream side of the filter, and acontroller having a regeneration determination unit for determining, bya comparison between the differential pressure detected at thedifferential pressure sensor and a determinative differential pressureas a threshold level for determination, whether or not a time, at whichforced regeneration is needed to burn the particulate matter captured onthe filter, has been reached, wherein: the controller comprises one thathas a variation determination unit for determining whether or not astate quantity relevant to an operation of the engine has variedabruptly and that, when the state quantity is determined to haveabruptly varied by the variation determination unit, performs processingto invalidate the determination by the regeneration determination unitduring a predetermined time in which an effect of the state quantity isconsidered to diminish to a negligible extent.
 2. The exhaust gaspurification system according to claim 1, wherein: the controller has afirst computing unit for computing a flow rate of exhaust gas, and asecond computing unit for computing the determinative differentialpressure based on the flow rate of exhaust gas as computed at the firstcomputing unit and a map preset in the controller and indicatingcorrelations between flow rates of exhaust gas and determinativedifferential pressures.
 3. The exhaust gas purification system accordingto claim 2, wherein: the exhaust gas purification system is furtherprovided with: a fuel control unit for controlling an injection quantityof fuel to be fed to the engine, an intake air volume sensor fordetecting a volume of intake air to be fed to the engine and outputtinga detection signal to the controller, an intake air temperature sensorfor detecting a temperature of intake air and outputting a detectionsignal to the controller, and an exhaust gas temperature sensor fordetecting a temperature of exhaust gas from the engine and outputting adetection signal to the controller; the controller further comprises: afuel injection quantity instruction unit for outputting an instructionsignal to instruct the injection quantity of fuel to the fuel controlunit, and an intake air weight computing unit for computing a weight ofintake air based on a density of intake air, which is determinedaccording to the temperature of intake air as detected at the intake airtemperature sensor and a map preset in the controller and indicatingcorrelations between intake air temperatures and intake air densities,and the volume of intake air as detected at the intake air volumesensor; an exhaust gas weight computing unit for computing a weight ofexhaust gas based on the weight of intake air as computed at the intakeair weight computing unit and the injection quantity of fuel asinstructed by the fuel injection quantity instruction unit; and thefirst computing unit of the controller performs processing to compute aflow rate of exhaust gas based on a density of exhaust gas, which isdetermined according to the temperature of exhaust gas as detected atthe exhaust gas temperature sensor and a map preset in the controllerand indicating correlations between exhaust gas temperatures and exhaustgas densities, and the weight of exhaust gas as computed at the exhaustgas weight computing unit.
 4. The exhaust gas purification systemaccording to claim 3, wherein: the exhaust gas purification system isfurther provided with: an engine speed sensor for detecting a revolutionspeed of the engine and outputting a detection signal to the controller;and the state quantity relevant to the operation of the engine is atleast one of the revolution speed of the engine as detected at theengine speed sensor, the injection quantity of fuel as instructed by thefuel injection quantity instruction unit, the volume of intake air asdetected at the intake air volume sensor, and the flow rate of exhaustgas as computed at the first computing unit of the controller.